While performing intercourse "doggie-style" this maneuver involves the pitcher inserting his curled finger into the catcher's mouth and pulling back to degrade the subject or cause pain.
A "double fish hook" involves the same maneuver using both fingers.
A "rusty fish hook" involves fingering the ass of the catcher prior to insertion of the finger(s) into the mouth.

When I walked in I saw Carlos spanking your sister's ass and giving her the fish hook.

While behind someone inserting your index finger into thier mouth and pulling back towards the back of thier head therefore catching them like a fish on a hook. The sexual refrences above are foreign to me. To my knowledge no one has ever caught a fish with a hook to the ass.

Amongst other things the fishhook is illegal in the U.F.C. because it is so brutally painfull.

Putting a finger in someones mouth while trying to hurt them and then pulling as hard as you can to rip their cheek. This resembles a fish being hooked so is called the fish hook.
As seen in The Gangs Of New York

A fighting move. Applying pressure with your thumb underneath and behind the jaw. Not effective under the chin. You really want to get your thumb in behind the jawbone and hold the pressure as long as neccesary. Excellent if someone has you in a hold. Less permanently damaging than gouging their eye out.

"Mike's in a choke hold. It doesn't look like he can get out of this! But wait! He just used the fish hook on his attacker! He's out!

when you're getting head from a chick and you have a dick piercing. just before you blow you catch the piercing onto her cheek and reel 'er in and watch her squirm and squeal. that is what i call "catch of the day"

Man, last night I fish hooked this bitch last night and damn that was a good catch.